Dynamics of Crystals with Molecular Impurity Centers. II. Resonance Scattering of Lattice Waves

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Abstract
We consider a monatomic host lattice containing molecular impurity centers and find that even in the harmonic approximation impurity modes can interact. This leads to an interference term in the scattering cross section for lattice waves which may be of the same order as the direct terms. From a knowledge of the nonvanishing elements of the scattering matrix, one can obtain all information on the possible scattering mechanisms and acoustical activity (depolarization) of the modes involved. A model calculation reveals the contribution to the forward-scattering cross section of the term due to the interference between a librational mode and the motion of the center of mass. We also study the case where the symmetry at the defect site is reduced. We conclude with a discussion of what we might expect in a more realistic situation and point out that from corresponding experimental data it should be possible to obtain information about the extent to which a molecular defect of lower symmetry affects the dynamics of the host lattice.

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